Allan Variance - Stability of Diode-Laser Spectrometer for Monitoring of Gas Concentrations

It has been proven that the characteristic of laser frequency stability is critically important for evaluation of metrology system performance when the function principle is based on laser-diode spectrometry. Trace gas measurements were calculated in the frequency domain and consequently recalculated to the time domain for the Allan variance expression. Detection of stable molecule unstable specie radical CN and OCS molecule has been compared. The Allan variance is mathematically defined, and then its behavior is extensively tested on experimental data. These are used to obtain modified and overlapping Allan variances in a dependency on the averaging time. Simultaneously, the integral formula for the direct conversion from the frequency to the time domain is used to get results for comparison. The stability analyses were determined by 2000 s measurements. Results show the possibilities of the presented procedure that is going to be used for the evaluation of direct time domain laser frequency stability measurements. Results are focused on the area of influence of the reactivity on the optimal averaging time for the minimum detectable concentration is demonstrated where a question of the frequency stability is very important.